A. Field of the Invention
This invention relates to the field of electronics, and particularly to solid state electronic circuits for receiving input from an array of photo detectors.
B. Description of the Prior Art
The input circuit associated with a photodiode array has to satisfy several requirements. The ac input impedance of the circuit should be low to assure constant bias voltage for a given diode with varying illumination. The bias voltage of each diode in the array should be independent of the physical properties (threshold voltage, etc.) of the input device in order to assure approximately the same bias voltage for each diode. Additionally, for compact and inexpensive realization, the circuit must be simple and suitable for construction as a metal oxide semiconductor (MOS) integrated circuit.
Prior art input circuits do not accomplish the above requirements. Such prior art circuits utilize a field effect transistor (FET) having a constant gate voltage as a tap gate for each of the photodiodes. A shortcoming of such prior art circuit is that the photodiode's anode potential is determined, for small diode currents, by the threshold voltage of the tap gate FET. Since the threshold voltage can vary appreciably (by as much as 50 mV) along the focal plane, the photodiode bias changes from individual photodiode to individual photodiode. Such change in bias causes a significant amount of fixed-pattern noise in the system.